Answer
The difference in tension between the two ends of the cable is 60 N
Work Step by Step
We can find the acceleration of the block:
$a = \frac{v-v_0}{t}$
$a = \frac{(4.0~m/s)-0}{2.0~s}$
$a = 2.0~m/s^2$
The tension at the end of the cable attached to the block provides the force to accelerate the block. We can find the tension at this end of the cable;
$T = ma$
$T = (20~kg)(2.0~m/s^2)$
$T = 40~N$
At the other end of the cable, the tension is equal to the force pulling on the cable. Therefore, the tension at that end of the cable is 100 N.
The difference in tension between the two ends of the cable is 60 N.